1. Field of the Invention
This invention relates to zinc phosphate coatings for metal surfaces and to processes for phosphatizing a metal surface with acidic aqueous phosphate solutions. The invention is applicable to a variety of substrates including cold rolled steel ("CRS"), zinc alloys, and aluminum.
2. Statement of Related Art
Present day phosphate coating solutions are generally dilute aqueous solutions of phosphoric acid and other chemicals which, when applied to the surface of an active metal, react with the metal surface to form on the surface of the metal an integral layer of a substantially insoluble amorphous or coating. Generally, the crystalline coatings are preferred.
Typically the solutions include phosphate ions, zinc and other metal ions, especially manganese, nickel, and other divalent metal cations, to provide specific characteristics desired in the final coating. Other ions typically present may be nitrate, nitrite, chlorate, fluoborate or fluosilicate. A typical phosphating process comprises the following sequence of process steps: (1) cleaning and conditioning; (2)phosphating itself; and (3) post treating. Rinses are generally employed between each of the noted steps to prevent or at least reduce carry over of materials to the next step.
Despite advances in both the composition of the phosphate coating solution and the phosphating process, there is a continued demand for still further improvements in the compositions and processes, in order to provide more control over the process, assure adequate coating weights, reduce formation of scale or white spots, and reduce adverse environmental impact and safety hazards.